1. Field of the Invention
The present invention relates to a demodulator and phase compensation method, and more particularly, to a demodulator and phase compensation method capable of reducing effectively phase fluctuation incidental on semi-synchronous detection in a digital radio communication.
2. Description of the Related Art
Heretofore, semi-synchronous detection is used as a demodulator of a digital radio communication. In the semi-synchronous detection, since a local oscillation (LO) frequency is not completely identical to a carrier frequency of a received signal, phase rotation (phase fluctuation) according to these frequency differences remains in a base-band signal after demodulation.
If the phase rotation remains, a transmitting data can not be recovered correctly (demodulation errors occur). Correspondingly, as technology of eliminating this phase rotation, a carrier recovery loop is used in general.
However, in the conventional carrier recovery loop, there is a problem that the phase rotation cannot fully be eliminated.
As an example which solves such a problem, there is a demodulator which includes a phase compensation loop (feed-forward loop) of cascade arrangement in addition to the conventional carrier recovery loop (feedback loop) (see, JP 2002-111766).
The conventional demodulator detects how much a demodulated signal with output side of the carrier recovery loop is shifted from a predefined phase position in rectangular coordinate, in the feed-forward phase compensation loop, and compensates for rapid phase fluctuation which takes place at the time of line switching to backup line from active line. That is, the conventional demodulator eliminates the rapid phase fluctuation which cannot be eliminated by the carrier recovery loop with subsequent feed-forward phase compensation loop.
Operation of the conventional demodulator is performed so that a phase detector detects the phase fluctuation at the time of line switching and an endless phase shifter adds reverse phase rotation to the demodulated signal with the phase compensation value to compensate for the phase fluctuation.
However, phase detector output contains generally a noise component (phase noise) incidental on phase detector itself regardless of phase fluctuation of the received signal. Therefore, in the state where there is no line switching, while the feed-forward phase compensation loop has been in a state of operation, the phase noise remains and there is a problem of degrading the demodulation characteristic on the contrary.
Here, a relation between loop bandwidth and carrier-to-noise power ratio (C/N) in a carrier synchronous operation limit is explained. FIG. 3 is a diagram showing a relation between loop bandwidth and carrier-to-noise power ratio (C/N) in carrier synchronous operation limit.
Referring to FIG. 3, as responsivity to the received signal becomes slow (loop bandwidth is narrow), C/N degradation by carrier jitter (fluctuation) decreases, and a carrier synchronization can be hold to a low C/N of a received signal. Meanwhile, as the responsivity to the received signal becomes fast (a loop bandwidth is wide), the phase noise is suppressed and C/N degradation by the carrier jitter becomes more dominant.
Next, a relation between loop bandwidth and C/N in a bit error rate (BER) characteristic is explained. FIG. 4 is a diagram showing a relation between loop bandwidth and carrier-to-noise power ratio (C/N) in bit error rate (BER) characteristic.
Referring to FIG. 4, as well as FIG. 3, as loop bandwidth narrows, an influence of carrier jitter becomes small. Meanwhile, as loop bandwidth is wide, C/N degradation by phase noise becomes small, but C/N degradation by the carrier jitter becomes large on the contrary.
According to the above explanation, when a loop bandwidth is narrow, a signal noise component received from the outside of a loop is suppressed, meanwhile, when a loop bandwidth is wide, a noise component generated within a loop is suppressed.
In the case of the conventional demodulator mentioned above, though the feed-forward phase compensation loop can compensate for the rapid phase fluctuation (demodulation errors) which takes place at the time of line switching to backup line from active line, there is no consideration to reducing the noise component (phase noise) incidental on phase detector itself.
Therefore, when the feed-forward phase compensation loop is effective in a state without line switching, the conventional demodulator mentioned above has the problem of degrading the demodulation characteristic on the contrary.